A clear sky at night
... is an astronomer's delight.
The only tricky bit is: knowing when we'll get one.
Recently, I've been keeping track of "official" data from my local Met Office's website and looking at the amount of time
the days are sunny and the nights are clear. It does seem to corroborate my gut-feeling - at least for the data I've got since the start of 2010. This shows that the spring/early-summer had significantly more clear nights (the black bar in the chart) than other seasons (at least for 2010 and 2011, when this piece was written).
When's the best time of year for observing?
|
Oddly enough, not during the summer. The main reason (apart from the unreliability of the british weather) is that the further north you go from the equator the bigger the difference between the length of daylight in the summer and the winter. So in equatorial areas, there is little difference in the number of daylight hours between the two seasons, whereas once you get into the polar (or southen polar, for that matter) regions you can find "the land of the midnight sun" when at certain times of the year the sun is above the horizon for most of the 24-hour day. In southern britain from late May to early August, the sun is still below the horizon at night - but not by much. What that means is the sky never gets really dark during those weeks. It's even worse for people living further north as the period of bright-nights is even longer.
The chart of the right shows when the Sun is up and how bright the Moon is (there's a larger version on my astronomy site along with charts for other locations, to compare with. See here .
As you can see, during the summer the evening twilight joins up with the pre-dawn lightening that masks out the dimmer astronomical sights. Though it could be argued that for most people, light pollution has already done the damage so there's not much more lost to bright nights.
Personally, I've found that springtime often offers the best nights. It's not too cold, there are a reasonable number of dark hours in a night (especially before the clocks go back, so you're in bed longer in the mornings) and for some reason the weather seems to be less cloudy, too. When I was doing my observing from England, I kept an unofficial (meaning I didn't always record when I was out - or added entries later) journal and came to the conclusion that in a typical year, accounting for weather, bright moon, social activities, getting up early for work and very occasionally something good on TV, I was getting between 15 and 20 nights observing in a year. There seemed to be a lot more entries during March and April than in other months and the only real runs of many clear nights in a row seemed to happen during springtime. |
 |
the days are sunny and the nights are clear. It does seem to corroborate my gut-feeling - at least for the data I've got since the start of 2010. This shows that the spring/early-summer had significantly more clear nights (the black bar in the chart) than other seasons (at least for 2010 and 2011, when this piece was written).